ui: scaffold packages/ui with theme foundations (tokens.css, useVscodeTheme)

[EhabY]

Tracked in DEVEX-624 · Project: Workspaces webview + UI library

---

Create the shared component library package. Mostly boilerplate + foundations; no components yet.

Scope

• New packages/ui (React + TS), wired like existing packages (vite, tsconfig, catalog deps, Storybook via @repo/storybook-utils)
• tokens.css: semantic token names mapped to --vscode-* variables (single place to theme-match VS Code, incl. high contrast)
• Move useVscodeTheme from packages/webview-shared into packages/ui (re-export from webview-shared for existing consumers during transition)
• package.json shaped for a future standalone NPM split: no workspace:* runtime deps, react as peer dependency, proper exports map (JS, tokens.css, codicon assets)

Out of scope

• Any actual components (follow-up issues)

Acceptance

• packages/ui builds, typechecks, and renders a trivial story in Storybook with tokens applied
• Package has zero runtime deps besides react (peer) at this stage

[linear-code]

ENG-3045

[EhabY]

This GitHub issue tracks DEVEX-624. The auto-created ENG-3045 has been canceled as a duplicate.

Project: Workspaces webview + UI library

[EhabY]

Consolidation update:

• The Linear-GitHub sync auto-created ENG-3045 from this issue and bi-directionally links them (closing one closes the other).
• ENG-3045 is marked as a sync stub — do not track work there.
• The canonical tracker is DEVEX-624, which ENG-3045 is related to.
• Project: Workspaces webview + UI library